The invention relates to a diecasting die and a diecasting method for sprueless diecasting, in particular in a diecasting hot-runner system, wherein the diecasting die is provided in a feeding region for forming a plug of solidified molten material that interrupts a flow of the molten material and can be completely remelted.
The sprue, which solidifies in the runners between the diecasting die and the casting mould in conventional diecasting processes, causes an additional material effort that usually amounts to 40% to 100% of the cast part. Even when the sprue is remelted for material recycling it still results in losses of energy and quality. The sprueless diecasting method avoids these disadvantages.
The sprueless diecasting method requires the liquid molten material to be either taken from the smelter to the mould and back for each cast or to be maintained in this state directly near the mould. The latter is achieved by the hot-runner method where all runners up to the mould are heated so that the molten material is kept in liquid state and at the same time prevented from flowing back into the smelter.
The backflow into the smelter can be prevented by valves, but also by a plug of solidified molten material which closes the gate in the diecasting die.
Devices and methods for sprueless diecasting to the formation of a plug of solidified molten material that seals a feeding region against a flow of molten material and can be remelted are known to the state of the art. Such devices and methods are described in particular for the diecasting of non-ferrous metals and especially plastics.
The publication DE 19846710 B4, which covers the injection moulding of plastics, envisages a heat withdrawal on the die opening, which would cause the molten material in this area to solidify. This prevents the molten material from flowing back into the runners and into the smelter.
A specific control of the remelting process of the solidified molten material, however, is not designed. Instead, the formed plug cannot be removed until the demoulding of the cast part.
Furthermore, the EP 1201335 A1 describes a hot-runner method for non-ferrous metals with a heated feeding tip, the feeding region, in which the molten material backflow into the runners and into the smelter is prevented by a plug in the unheated die tip. The feeding tip is heated on the outside. The plug comes off the wall of the feeding tip and is forced out of the die tip by the molten material that is injected in the next casting.
A receiver for plug is required to prevent the solid plug from being hurled into the mould. However, this would interfere with the molten material flow during the injection. Since the material is injected into the mould at a speed of 50-100 m/s the mould could also be damaged by a loose plug that is dragged along by the molten material. It is not possible to fully remelt the plug in a controlled way. Even if one would try to solve this problem, the solution would require very long cycle periods affecting productivity negatively.
The object of the publication DE 4319306 A1 is a sprueless injection moulding method for synthetic resins. For this purpose an injection moulding device with a feeding runner equipped with a tip heating was designed. Though the injection nozzle has a two-section heating—separated for the nozzle body and the nozzle tip—for a specific control of the molten material, the heating again takes place through the injection nozzle wall, which delays the heat provided by the heating in reaching the molten material. Furthermore, an additional valve pin is required to control the molten material inflow since no sealing plug is formed. On top of that, this system is only suitable for the injection moulding of plastics as the installation of a heating system in the nozzle tip would either decrease its compressive strength in such manner that it could not sustain the pressures present during the diecasting of metals, or would result in a massive nozzle tip structure that increases the thermal inertia of the material between the heating and the molten material and thus causes very long cycle periods.
The subject of the publication DE 3809643 is the sprueless injection moulding of synthetic resins. For this method, the gate is sealed by cooling it, and is re-opened by reheating it. However, this requires a complex gate which would allow for both a heating of the resin in the nozzle as well as a heat transfer to a cooling medium.
The publication DE 3531127 A1 describes an element which enables the gate of an injection moulding nozzle for resins to be sealed and opened by means of thermal effects. On the tip of the element, the molten material is solidified by means of cooling so that it seals the gate. When the tip is heated by a heating element built into the element, the material melts again. The cooling is ensured by not transferring any more heat to the tip of the element after the heating element is turned off and by dissipating the present heat from the tip. This eliminates the need for additional cooling equipment.
However, the element must be built into the gate additionally. Furthermore, the heating is placed in the element, which results in delayed heat dissipation from the inside to the outside and affects the speed of the injection moulding process adversely. The hollow internal space reserved for the heating element affects the compressive strength, which becomes a problem especially for the diecasting of molten metals.
The publication DE 2542875 also covers a solidified plug in the nozzle tip for the injection moulding of thermoplastics with the objective of sealing it. The remelting is achieved by the further flow of heat from the nozzle body; however, additional heating and cooling elements were designed as well.
In this case, too, the heating is designed to take place from outside of the nozzle, which increases the response time of the process and which cannot control the liquefaction resp. solidification of the molten material in the nozzle sufficiently—despite envisaged temperature sensors.
Though the sealing of a nozzle by a plug of solidified molten material is known to the state of the art for injection moulding and diecasting of different materials, especially for the sprueless injection moulding—and it is known that this plug can be remelted by means of heating—the state of the art also indicates that all attempts at bringing the temperature influence element as close to the molten material as possible with the intention to increase the speed and controllability of the casting process, have amounted to nothing more than bringing the indirect heating close to the molten material with the separation of a wall between the two still existent. A fast casting process and a high thermal output on the feeding point cannot be achieved with the known devices, in particular because an increasing pressure—in the interest of short cycle periods—would also require an increased thickness of the diecasting die, which would in turn increase the inertia further.